Field of the Invention
This invention relates to liquid compositions containing solubilized polyorganosiloxanes. More particularly, this invention relates to liquid compositions containing relatively low concentrations of organosiloxane block copolymers. Using these compositions, thin coatings of the copolymers can be applied to a variety of substrates, including polydiorganosiloxane elastomers and organic polymers.
The prior art teaches that liquid organic hydrocarbons such as toluene are useful solvents for preparing organosiloxane block copolymer resins as well as vehicles from which coatings of the resins can be applied to silicone rubber. U.S. Pat. No. 3,639,155 to R. C. Hartlein and H. L. Vincent, which issued on Feb. 1, l972, discloses that the copolymers described in this patent are soluble in the organic liquids conventionally used for silicones, such as aromatic hydrocarbons, chlorinated aliphatic hydrocarbons, and ketones. Useful solvents include toluene, xylene, benzene, methylisopropyl ketone and trichlorofluoromethane. Aromatic hydrocarbons such as toluene and xylene are relatively inexpensive compared to many of the other organic liquids disclosed in the aforementioned Hartlein and Vincent patent, and for this reason would be among the preferred solvents for use in preparing coating compositions.
U.S. Pat. No. 3,280,214, which issued to D. Mitchell on Oct. 18, 1966, describes room temperature curable organosiloxane block copolymers containing blocks of from 5 to 400 dimethylsiloxane units and blocks formed by the hydrolysis of organotrihalosilanes in which the organic group bonded to silicon is aryl, alkaryl, or haloaryl. A compound having at least two silicon atoms bonded to three halogen atoms functions as a coupling agent between the dimethylsiloxane and monoarylsiloxane blocks of the final copolymer.
The block copolymers disclosed by Mitchell and others such as Vincent in U.S. Pat. No. 3,328,481, which issued on June 27, 1967, contain substantial concentrations of phenylsiloxane units to achieve higher levels of heat and oxidation resistance relative to polymers containing only methylsiloxane units. Mitchell teaches that the phenyl-containing siloxane units constitute up to 95 mol % of the siloxane units present in the copolymer. While the presence of phenyl or other aryl radical bonded to silicon is desirable for imparting heat stability to the copolymer, reactants containing these radicals may not be compatible with the polydimethylsiloxanes often employed as the coreactant for preparing the copolymer. The incompatibility of polydimethylsiloxanes and phenylsiloxane polymers was recognized by Mitchell in the aforementioned U.S. Pat. No. 3,280,214, which teaches that it is necessary to at least partially react a polydimethylsiloxane with the arylsiloxane components in the presence of a condensation catalyst to achieve a compatible mixture that can be further polymerized in an organic solvent such as toluene.
Room temperature curable organosiloxane block copolymers are also described in U.S. Pat. No. 3,629,228 to R. C. Hartlein and C. R. Olson, which issued on Dec. 21, 1971. The copolymers disclosed in this patent comprise blocks of diorganosiloxane units wherein at least 80% of the hydrocarbon radicals are methyl, organosiloxane blocks containing an avrage of from 1 to 1.3 hydrocarbyl radicals per silicon atom, at least 50% of the hydrocarbyl radicals being phenyl or other aryl radical and terminal or "endblocking" monoorganoalkoxysiloxane units located at the ends of repeating unit sequences. The polymer is cured by reaction of the alkoxy groups in the terminal units at ambient temperature in the presence of moisture and a catalyst composition that includes an organotitanate and an amine.
Any of the aforementioned room temprature curable organosiloxane block copolymers are useful coating materials for a variety of substrates, including silicone rubber. The copolymers described in U.S. Pat. No. 3,629,228 are unique by virtue of their unexpected resistance to accumulating dirt. This property makes the copolymers particularly useful as coating materials for silicone rubber, which has a tendency to attract dirt and assume an undesirable appearance.
Because the room temperature vulcanizable organosiloxane block copolymers disclosed in the aforementioned Hartlein and Vincent patent can be solids or relatively high viscosity liquid materials at ambient temperature, if one desired to apply relatively thin coatings of these materials, i.e., coatings less than about 5 mils (0.013 cm.) thick, it would usually be necessary to dilute the copolymer to a concentration level below 60% by weight. Applicant has found that at these concentration levels preferred organosiloxane block copolymers will not form coherent films on cured polydimethylsiloxane rubber. It is therefore not feasible to apply thin coatings of these copolymers at room temperature in accordance with the teaching of Hartlein and Vincent using liquid aromatic hydrocarbons as the solvent.
Japanese Patent Publication No. 82/5985 teaches that silicone rubber can be coated using polysiloxane compositions obtained by reacting a copolymer containing SiO.sub.4/2 and R.sub.3 SiO.sub.1/2 units, where R represents a monovalent hydrocarbon group, with a polydiorganosiloxane having a viscosity of from 10,000 to 2,000,000 centistokes (0.01 to 20 m.sup.2 /sec) at 25.degree. C. The resultant reaction product is dissolved in a mixture of a liquid hydrocarbon and a volatile organosilicon compound, which can be a linear or cyclic polydiorganosiloxane exhibiting a boiling point of from 70.degree. to 250.degree. C. under atmospheric pressure. In the preferred embodiments and examples of this publication, the rubber is a polydimethylsiloxane, the resin composition contains an aminoalkylsilane, and octamethylcyclotetrasiloxane is used as the solvent for the resin.
If one were to consider replacing the organosiloxane resin described in the examples of the aforementioned Japanese patent publication with a block copolymer containing phenylsiloxane units such as those described in any of the aforementioned U.S. patents, based on the teaching of the aforementioned patent to Mitchell, it would be reasonable to expect a liquid polydimethylsiloxane to be incompatible with the block copolymer. Mitchell advises that prolonged heating in the presence of a condensation catalyst may be necessary to obtain a homogeneous, single phase reactant from a phenyl-containing polysiloxane and a polydimethylsiloxane. Experimental data in the present specification demonstrate that an organosiloxane copolymer of the type disclosed in the aforementioned patent to Hartlein and Olson, which contains silicon-bonded phenyl groups, is incompatible with a polydimethylsiloxane exhibiting a viscosity of 5.times.10.sup.-6 m.sup.2 /sec.
The prior art does not teach suitable solvents for coating compositions having relatively low concentrations of organosiloxane block copolymers containing both dimethylsiloxane and phenylsiloxane units. Neither does the prior art teach solvents for the block copolymers which are compatible with substrate surfaces of both polyorganosiloxane rubbers, also referred to as silicone rubbers, and a variety of synthetic organic polymers in the absence of a coupling agent such as the aminoalkylsilane disclosed in the examples of the aforementioned Japanese patent publication.
It is therefore an objective of this invention to define a class of coating compositions containing up to about 60% by weight of solubilized organosiloxane block copolymers having more than about 20mol % of phenylsiloxane units in the polymer.
A second objective of this invention is to provide a method for applying relatively thin coatings of siloxane block copolymers to a variety of substrates, including silicone rubbers and synthetic organic polymers.